The present invention relates to fuel tank pressure control systems and, more particularly, to an automatic, pressure and temperature operated, vapor pressure control system for controlling vapor pressure and maintaining a sufficient vacuum in a fuel storage tank.
Enclosed tanks or containers which store all forms of gases, or liquids that expand or convert at varying temperatures into gases, share one basic problem: the internal vapor pressure must be controlled. Where the tank is used to store fuel to be pumped or otherwise removed from the tank, such as gasoline or diesel, it is desirable to maintain a sufficiently low vapor pressure relative to the external or "ambient" air pressure to create a vacuum in the tank in order to prevent the fuel vapor from being vented to atmosphere during the dispensing procedures. A relative vacuum also assists in the dispensing of the fuel from the tank by drawing outside air or, more commonly, vapor from the tank of a vehicle into which the fuel is being pumped, into the storage tank to displace the fuel being removed.
For instance, in a "gas station" type operation, fuel is stored in a tank which is in communication with a pump or other type of dispensing mechanism. Typically, the fuel in the tank is in both a liquid and vapor state. Since the storage tank has fixed dimensions, if the fuel is heated, e.g., by a temperature increase or by warm vapor from the tank of the vehicle being drawn into the storage tank during the dispensing procedure, it will expand causing an increase in vapor pressure within the tank. If the storage tank vapor pressure increases to a level above the ambient air pressure, it becomes difficult, if not impossible, to draw the vapor from the vehicle into the storage tank to replace the fuel being pumped. This objectionable variation in pressure is particularly aggravated in fuel storage tanks located above ground, which are subject both to changes in the ambient temperature and to direct sunlight transferring heat through the sides of the tank.
Known fuel tank vapor pressure control methods include the use of a pressure sensitive, relief valve mechanism incorporated into a wall of the tank, which is set to expel vapor from the tank if the vapor pressure becomes too great. The release of the vapor decreases the volume of fuel stored in the tank and, correspondingly, decreases the vapor pressure and temperature. By setting the release valve mechanism to expel vapor and maintain the tank vapor pressure at a predetermined level below the ambient air pressure, a vacuum will also be maintained.
In the case of flammable or environmentally harmful gases being expelled by the release valve mechanism, such as the case with gasoline vapor, it should be burned off immediately upon discharge from the tank. To do this, an open flame is provided downstream of the release valve opening to burn off the vapor. This adds cost, complexity and safety hazards, in addition to the lost fuel which is expelled and burned off, to the fuel storage and dispensing operation.
Most all fuels contain at least some amount of water. Thus, when the outside temperature is near, or below freezing, (i.e., approximately less than 35 degrees fahrenheit), there is no need for a tank vapor pressure control system, since maintaining a pressure vacuum under those conditions will cause the temperature of the fuel or liquid in the tank to be lowered below the freezing point of whatever water is present, causing it to freeze, thereby creating difficulties in the proper dispensing of the fuel. Because it is often the case that the external temperature will vary dramatically over a twenty-four hour period, i.e., cold during the night and hot during the day, the "burn off" element, i.e., the open flame, must either be maintained during the time period when no fuel is being expelled from the tank, thereby requiring an additional fuel supply, or it must be capable of being operated only when fuel is being expelled, thereby requiring an ignition system. In either case, additional cost, maintenance problems and complexity is added to the operation.
Thus, it is desirable to provide a system for fuel tank vapor pressure control which is capable of maintaining the tank vapor pressure at a level which provides an adequate vacuum for efficient dispensing of the fuel, without releasing any vapor from the tank. It is further desirable that such a system be capable of automatically operating only when the ambient temperature conditions so require.